Catalytic dehydrogenation of amine borane complexes

ABSTRACT

ABSTRACT A method of generating hydrogen includes the steps of providing an amine borane (AB) complex, at least one hydrogen generation catalyst, and a solvent, and mixing these components Hydrogen is generated. The hydrogen produced is high purity hydrogen suitable for PEM fuel cells. A hydrolytic in-situ hydrogen generator includes a first compartment that contains an amine borane (AB) complex, a second container including at least one hydrogen generation catalyst, wherein the first or second compartment includes water or other hydroxyl group containing solvent. A connecting network permits mixing contents in the first compartment with contents in the second compartment, wherein high purity hydrogen is generated upon mixing. At least one flow controller is provided for controlling a flow rate of the catalyst or AB complex.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/414,572, filed on Apr. 28, 2006, entitled “CATALYTIC DEHYDROGENATIONOF AMINE BORANE COMPLEXES” which is incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The U.S. Government may have certain rights to the invention based onNASA Grant No. NAG3-2751.

FIELD OF THE INVENTION

The invention relates to hydrogen generation systems, more specificallyto catalyzed amine borane (AB) complex-based hydrogen generation methodsand related systems.

BACKGROUND OF THE INVENTION

Hydrogen has been endorsed by numerous world leaders and decision makersin both public and private sectors and hailed as the key to a cleanenergy future. Many believe that the future alternative to currentfossil-based economy will involve hydrogen—used as the primary energycarrier in all energy sectors. In that regard, hydrogen will be utilizedmuch like electricity that is presently the defacto energy carrier inmuch of the world.

Hydrogen and electricity are complementary energy carriers and togetherthey can create an integrated energy system based on distributed powergeneration and use. Hydrogen and electricity are interchangeable using afuel cell (to convert hydrogen to electricity) or an electrolyzer (forconverting electricity to hydrogen). A regenerative fuel cell workseither way, converting hydrogen to electricity and vice versa.

The advantages of using fuel cells for power generation include: 1)ability to convert fuel into electricity directly; and 2) being anelectrochemical device, they are more efficient than the Carnot cyclebased energy conversion devices such as internal combustion engines(ICE).

As noted above, hydrogen can be used in fuel cells to power electricmotors. Furthermore, hydrogen, in combination with proton exchangemembrane fuel cells (PEMFC), can replace rechargeable batteries as thepower source of choice in certain applications, such as military andother applications requiring portable power.

Presently, rechargeable power sources are used for cell phones, PDAs,and laptop computers, among others. However, the majority of militaryapplications, particularly those involving soldier-portable devicesstill utilize standard, non-rechargeable primary batteries. The reasonfor this is that primary batteries offer long shelf life, robustness,and ease-of-use.

Secondary (rechargeable) batteries have improved over the years, but analkaline primary battery still delivers 50% more power than lithium-ion,one of the highest energy density secondary batteries in existence. Theprimary lithium battery used in cameras provides more than three timesthe energy of a secondary lithium-ion battery of same size. Moreover,rechargeable batteries are vulnerable to the elements as well as extremetemperatures, humidity, salt, and other exposure. While recentinnovations in the technologies used in rechargeable batteries hasreduced these risks, rechargeable batteries are not yet as powerful,rugged or reliable as primary batteries.

Despite their dependability and superior capacity, the one-time useconstraint of primary batteries increases their cost to over thirtytimes that of rechargeable batteries (mainly because new batteries areconstantly required to be replaced). Moreover, the logistics of gettingnew batteries delivered to military field units can be challenging oreven impossible. Soldiers are often required to carry up to 30 lbs ofbatteries in the field to power their electronic gear. Rechargeablebatteries would obviously be a much lighter alternative, but theirlimitations due to exposure make them too risky for many defenseapplications, even though they are still used in military trainingexercises.

Compounds represented by the empirical formula B_(x)N_(x)H_(y) have beenknown and used as high capacity hydrogen carriers. Generation systemsbased on such carriers can provide hydrogen storage for hydrogenon-demand applications. However, the hydrogen is often too impure forimportant applications, the process yield is low, the process requires ahigh temperature (e.g. greater than 100° C.) for reasonable yield,and/or environmentally harmful materials are required.

For example, U.S. Pat. No. 4,157,927 to Chew et al. discloses amineboranes and their derivatives mixed with heat producing compounds suchas lithium aluminum hydride or a mixture, such as NaBH₄/Fe₂O₃ ashydrogen generating formulations. The mixed powder is then pressed intopellets and ignited to generate hydrogen or deuterium. The formulationsdisclosed do not produce the ultra high purity hydrogen gas required byPEMFC and other demanding applications.

U.S. Pat. No. 4,381,206 to Grant et al. discloses an amine borane gasgenerating system comprising hydrazine bis-borane or its deuteratedderivatives in the form of a pellet, which serves as the thermalinitiator for further thermal decomposition. An all amine borane gasgenerator system which consists of N₂H₄2BH₃ and H₂B(NH₃)₂BH₄ provideshydrogen from a self-sustaining reaction after the self-sustainingreaction is initiated by a heat source such as a nichrome wire. Again,formulations disclosed by Grant do not produce the ultra high purityhydrogen gas required for the PEMFC applications.

Ammonia borane (AB) complex has the highest hydrogen content (about 19.6wt %)—highest amongst all amine boranes with a system-level H₂ energystorage density of about 2.74 kWh/L vs. 2.36 kWh/L for liquid hydrogen.At near room temperatures and atmospheric pressure, AB is a whitecrystalline solid, and is stable in both water and ambient air.

Thermolysis has been used as a method of choice to generate hydrogenfrom AB complexes. The drawbacks of thermolysis for hydrogen release areas follows: Ammonia borane pyrolysis begins at temperatures below 140°C. To release substantially all of the hydrogen contained in ammoniaborane complex, however, temperatures above 500° C. are required. Theoverall process is exothermic, but requires heat to be added to activatethe AB complex. The overall thermolytic reaction can be written asfollows:

NH₃BH₃+Heat→BN+3H₂

In practice, thermolysis of AB complex involves competing reactionsleading to the formation of certain undesirable by-products. Forexample, FTIR analysis of the evolved gases from the thermolysis of ABcomplex has shown that monomeric aminoborane (BH₂NH₂), borazine, anddiborane is also produced. The aminoborane comprises poly-(aminoborane),(BH₂NH₂)_(n).

Poly-(aminoborane), the inorganic analog of polyethylene, is anonvolatile white solid. Volatile compounds are undesirable impuritiesthat make hydrogen from direct thermolysis of AB complex unfit for PEMFCapplications. Furthermore, formation of these undesirable compoundslowers the yield of H₂ from direct thermolysis of AB complex.

Because direct high temperature thermolysis of amine borane complexes,in general, and AB complex, in particular, generates low yield ofinferior quality hydrogen contaminated with volatile pyrolysis products,it is highly desirable to find new energetically self-sufficientprocesses that can generate ultra high purity H₂ at low, near ambienttemperatures.

Hydrolytic or methanolic cleavage of amine borane complexes provides 3moles of hydrogen per mole of AB complex. Although this process has beenused in the field of modern synthetic organic chemistry and forpharmaceutical applications, only recently has it been applied as a wayof utilizing AB complex for the storage of H₂:

NH₃BH₃+3H₂O→NH₃+H₃BO₃+3H₂NH₃BH₃+3CH₃OH→NH₃+B(OCH₃)₃+3H₂

As mentioned above, AB complex is a stable adduct. Therefore, the abovehydrogen generating reactions have involved harsh acidic condition (e.g.refluxing in aqueous HCl) or the use of heterogeneous catalysts based onpalladium or nickel.

Using methanol as a reagent, such as in the reaction above, has a majordrawback as it increases the overall weight of the AB-based hydrogengenerator, thus reducing the overall specific H₂ energy storage densityof the hydrogen generator.

Published U.S. Application No. 20050180916 to Autrey et al. disclosesammonia borane deposited onto a support or scaffolding material at a 1:1weight ratio. Supports are porous materials such as mesoporous silica.Autrey also discloses a transition metal catalyst and a carbon support.This material exhibits hydrogen release rate that is about one order ofmagnitude greater than that from the neat compound. In addition, thedisclosed temperature for hydrogen release is reduced to about 85° C.

Thus, all known methods for low temperature hydrogen release from an ABcomplex and related materials have significant shortcomings. Thermolyticdehydrogenation requires a relatively high temperature and generates asignificant concentration of by-product impurities which make thisprocess unfit for certain important applications. Although Autrey et al.discloses dehydrogenation of AB complex at about 85° C., a special orcomplicated support structure is required and no more than two moles ofhydrogen are generated. Hydrolytic or related cleavage processes requireharsh chemicals, such as HCl in reflux temperatures. Therefore what isneeded is an energetically, self-sustaining method and apparatus forhydrolytic and/or thermolytic dehydrogenation of amine borane complexes,in a hydrogen generator for portable, on-board and off-board powergeneration via PEMFC and ICE.

SUMMARY

A method of generating hydrogen comprises the steps of providing anamine borane (AB) complex, at least one hydrogen generation catalyst,and a solvent, and mixing these components. The hydrogen produced by themixing step exclusive of any separation processing is generally “highpurity hydrogen gas”. As used herein, “high purity H₂ gas” refers to agas containing 99% by volume hydrogen, preferably a gas containing 99.9%by volume H₂, more preferably a gas containing 99.99% by volume H₂ (orhigher).

The hydrogen generation catalyst can be selected from cobalt complexes,noble metal complexes and metallocenes. For example, the noble metalcomplex can be selected from Na₃RhCl₆, (NH₄)₂RuCl₆, K₂PtCl₆,(NH₄)₂PtCl₆, Na₂PtCl₆ and H₂PtCl₆.

In one embodiment, the amine borane complex can be NH₃BH₃. The method isgenerally performed in a temperature range of between 25° C. and 80° C.

The solvent can comprise a hydroxyl group comprising solvent whichparticipates as a reactant in generating hydrogen. This embodiment isreferred to as hydrolytic dehydrogenation. For example, the hydroxylgroup comprising solvent can be water or methanol. In this embodimentthe amine component of the AB complex can be an organic amine. Thehydrolytic method can include the step of adding an amine-sequesteringagent, such as polyacidic acids selected from malic, malonic, oxalic,chromic, phosphoric, pyrophosphoric and sulfuric.

The solvent comprises a coordinating solvent. This embodiment isreferred to as thermolytic dehydrogenation. The coordinating solvent canbe selected from mono, di, tri, and tetraglyme, tetrahydrofuran,dimethylformamide, 1-Methyl-2-Pyrrolidinone, crown ethers andethylenediamine. In the thermolytic embodiment, the catalyst cancomprises a salt, such as the salt of a wealk base and strong acid, orammonium halide salts, such as NH₄I, NH₄Br or NH₄Cl, plus thosedescribed for hydrolysis.

A hydrolytic in-situ hydrogen generator comprises a first compartmentthat contains an amine borane (AB) complex, a second containercontaining at least one hydrogen generation catalyst, wherein the firstor second compartment includes water or other hydroxyl group containingsolvent. A connecting network is provided for mixing contents in thefirst compartment with contents in the second compartment, whereinhydrogen is generated. At least one flow controller controls a flow rateof the catalyst or AB complex. In one embodiment the first compartmentcontains dry AB complex, and said second compartment contains thecatalyst and water. In another embodiment, the first compartmentcontains the AB complex in water, and the second compartment containsthe catalyst.

A Proton Exchange Membrane Fuel Cell (PEMFC) based electrical generatorcomprises an ion-exchange membrane interposed between an anode and acathode to form a membrane/electrode assembly (MEA), the MEA beinginterposed between a fuel gas diffusion layer and an oxidant gasdiffusion layer. An oxidant flow network is in fluid connection with gasdiffusion layer, the oxidant network having an input portion forsupplying oxidant, and a fuel flow network fluid in fluid connectionwith the fuel gas diffusion layer. The fuel network has an input portionfor supplying fuel, wherein the fuel flow network is fluidly connectedto a hydrolytic in-situ hydrogen generator. The generator comprises afirst compartment that contains an amine borane (AB) complex, a secondcontainer containing at least one hydrogen generation catalyst, whereinthe first or said second compartment includes water or other hydroxylgroup containing solvent. A connecting network is provided for mixingcontents in the first compartment with contents in the secondcompartment, wherein hydrogen is generated upon mixing. At least oneflow controller controls a flow rate of the catalyst or AB complex.

In one embodiment, the generator further comprises a heat exchanger. Theheat exchanger is thermally coupled to the fuel cell and the hydrogengenerator, wherein the heat exchanger receives heat generated by thefuel cell and transfers heat to the hydrogen generator. The heattransferred is generally sufficient to avoid the need for an auxiliaryheating device for the hydrogen generator. The hydrogen produced by thehydrogen generator is generally high purity hydrogen gas. The hydrogengeneration catalyst can be selected from cobalt complexes, noble metalcomplexes and metallocenes. The noble metal complex can be Na₃RhCl₆,(NH₄)₂RuCl₆, K₂PtCl₆, (NH₄)₂PtCl₆, Na₂PtCl₆ or H₂PtCl₆.

BRIEF DESCRIPTION OF THE DRAWINGS

There is shown in the drawings embodiments, which are presentlypreferred, it being understood, however, that the invention can beembodied in other forms without departing from the spirit or essentialattributes thereof.

FIG. 1 is the schematic of an exemplary electrical power sourcecomprising a hydrogen generator according to the invention coupled to aPEM fuel cell.

FIG. 2 shows the extent of hydrogen generated by mixing 0.4 g (12.8mmol) of AB and 0.2 mL of water. 1.0 mL of a 20 mM solution of H₂Cl₆Ptwas added with an injection rate of 0.05 mL/min. The flask was insulatedwith glass wool.

FIG. 3 shows the comparative hydrogen generation results obtained—forthe insulated (with glass wool) isothermal (by immersion in an oil bathat 30° C.), and heated for the first 5 minutes at 35° C. reactorarrangements. To 0.3 g (12.8 mmol) of as received AB (90% technicalgrade) and 0.1 mL of water, 0.44 mL of saturated solution of K₂Cl₆Pt wasadded using an injection pump set at 0.02 mL/min.

FIG. 4 shows the hydrogen generation results obtained using 0.12 g of ABwith 0.25 mL, 0.35 mL, and 0.5 mL of 15.0 mM solutions of Cl₆K₂Pt addedat once.

FIG. 5 shows the comparative hydrogen generation results obtained fromthe hydrolysis of AB complex using PGM catalysts such as K₂PtCl₆,(NH₄)₂RuCl₆, Na₃RhCl₆, and (NH₄)₂PtCl₆. Ruthenium and rhodium salts seento be more active than platinum and rhodium salts.

FIG. 6 shows the hydrogen generation results obtained from thehydrolysis of borane tertbutylamine complex using the PGM catalystsK₂PtCl₆, (NH₄)₂RuCl₆, (NH₄)₂PdCl₆, Na₃RhCl₆, and (NH₄)₂PtCl₆.

FIG. 7 shows hydrogen generation results obtained from mixingapproximately 0.10 g of the amine borane complex, 0.34 g of a cocktailwith the composition: 3.3 g H₂O, 2.5 g H₃PO₄ (85% by weight in water),and 0.025 g of a Na₃RhCl₆ catalyst, added at room temperature.

FIGS. 8 and 9 show kinetics of AB hydrolysis using ¹¹B-NMR obtained byadding 0.01 g of AB to 7 mL of 5 mM, 10 mM, 30 mM, and 45 mM K₂Cl₆Pt inD₂O solution at 25° C., 30° C., and 35° C. FIG. 8 shows the reactionorder with respect to the catalyst. FIG. 9 shows the initial rate of ABhydrolysis using 10 mmol K₂Cl₆Pt catalyst at 30° C.:υ_(o)=k[Cat]^(1.4)[AB], where, [Cat] and [AB] denote the concentrationof the catalyst and AB complex, respectively.

FIG. 10 shows the temperature dependence of the AB hydrolysis reactionrates given by the Arrhenius equation: ln k=ln A−E_(a)/RT, where,activation energy (E_(a)) is equal to: 83.9 kJ mol⁻¹ and pre-exponentialfactor (A) is equal to: 5.3×10¹⁰ L mol⁻¹ s⁻¹.

FIG. 11 shows the amount of hydrogen evolved as a function of time fordata from catalyst cocktail injection rates 0.025 mL/min and 0.05mL/min. 0.5028 g of AB complex (Aldrich 90%) was placed in an insulatedvial and mixed with 1.2908 g of Na₃RhCl₆-H₃PO₄ catalytic cocktail.

FIG. 12 shows the amount and rate of hydrogen generation as a functionof time for the data obtained by dropping three tablets of AB complex(Aldrich 90%), one at a time, into an isothermal hydrolysis reactor keptat 80° C. in an oil bath. The reactor vial contained 1.5 mL of acatalytic cocktail mixture with following approximate composition:water/phosphoric acid/Na₃RhCl₆ salt=57.07/42.57/0.36 percent by weight.

FIG. 13 shows the amount of thermolytic hydrogen generation as afunction of time for the data obtained by placing 0.205 g of NH₃BH₃(Aldrich 90%) inside a 10 mL glass vial reactor containing 0.4 mL of2-methoxyethyl ether at 70° C.

FIG. 14 shows the amount of thermolytic hydrogen generation as afunction of time for the data obtained by placing 0.204 g of NH₃BH₃(Aldrich 90%) inside a 10 mL glass vial reactor containing 0.4 mL of2-methoxyethyl ether and 0.0405 g NH₄I at 70° C.

FIG. 15 shows the amount of thermolytic hydrogen generation as afunction of time for the data obtained by placing 0.203 g of NH₃BH₃(Aldrich 90%) inside a 10 mL glass vial reactor containing 0.4 mL of2-methoxyethyl ether, 0.0065 g K₃Co(CN)₆, and 0.0074 g NH₄I at 70° C.

DETAILED DESCRIPTION

A method of generating hydrogen comprising the steps of providing anamine borane (AB) complex, at least one hydrogen generation catalyst,and a solvent, and mixing the AB complex, the catalyst and the solvent,wherein H₂ is generated by the reaction. The reaction can proceed atcomparatively low temperatures, such as 50-80° C., and yield in excessof two moles of hydrogen. The catalytic dehydrogenation can proceed viaa hydrolytic or thermolytic route. In the case of thermolyticdehydrogenation, no strong acids are required. As used herein,hydrolytic dehydrogenation refers to a process which includes a hydroxylgroup comprising reagent, such as, but not limited to, water ormethanol. For both hydrolytic or thermolytic dehydrogenation accordingto the invention no special or complicated support structures arerequired.

The hydrogen generated is high purity H₂ gas, suitable for demandingapplications such as PEMFC. As noted above, an issue with the directpyrolytic dehydrogenation of AB complex based on related work isco-production of undesirable compounds such as borazine, monomericaminoborane, and diborane that are known to adversely affect performanceof the proton PEMFC.

Although generally described herein related to PEMFCs, other fuel celltypes can also utilize hydrogen generated according to the invention.Such fuel cells include alkaline fuel cells, phosphoric acid fuel cells,molten carbonate fuel cells, and solid oxide fuel cells.

The AB complex can comprise a variety of amine borane (AB) complexes.For hydrolytic dehydrogenation, (AB) complexes having an organic aminecomponent, such as borane dimethylamine, borane morpholine, and boranetertbutylamine have been found to be effective for generating hydrogen.Ammonia borane (NH₃BH₃) can be used for both hydrolytic and thermolyticdehydrogenation according to the invention. In a preferred embodiment,the AB complex is ammonia borane (NH₃BH₃), referred to by some asammonia-borane complex and others as borane-ammonia complex. As notedabove, ammoniaborane complex is the simplest of amine borane complexesand is a crystalline solid that contains 19.6-wt % hydrogen.

As used herein, a “hydrogen generation catalyst” is any material thatincreases the rate of hydrogen release from the AB complex in thepresence of the particular solvent(s) at the particular temperature ofoperation. Suitable catalysts generally include transition metalcomplexes including Co complexes, noble metal complexes andmetallocenes. Regarding noble metal comprising catalysts, exemplarycatalysts are generally based on the platinum group metals (PGM),comprising iridium, osmium, palladium, platinum, rhodium, and ruthenium.Such catalysts include Na₃RhCl₆, Chlorotris(triphenylphosphine)rhodium(I), (NH₄)₂RuCl₆K₂PtCl₆, (NH₄)₂PtCl₆, Na₂PtCl₆, and H₂PtCl₆.Metallocenes can include Fe(C₅H₅)₂ (ferrocene) anddi-μ-chlorobis(p-cymene)chlororuthenium (II). Exemplary transition metalcomplexes including Co can include K₃Co(CN)₆, and Co(NH₃)₆Cl₃. In thecase of thermolytic dehydrogenation, the hydrogen generation catalystscan also comprise salts, including salts with weak base and strong acidcharacteristics, such as ammonium halide salts, including NH₄I, NH₄Br,and NH₄Cl.

For hydrolytic dehydrogenation the solvent can be a hydroxyl groupcontaining compound, such as water or methanol. For thermolyticdehydrogenation the solvent is preferably a coordinating solvent withchelating effects, selected from the group consisting of mono, di, tri,and tetraglyme, tetrahydrofuran, dimethylformamide,1-methyl-2-pyrrolidinone, crown ethers and ethylenediamine.

Regarding the thermolytic dehydrogenation of ammonia borane, theInventors have found that if the ammonia borane is simply heated at 70°C. for even several days, no detectable hydrogen gas is released. If anon-coordinating solvent such as iso-octane is added, 0.7 moles of H₂per mole of AB complex are generated in a period of one day. If a weaklycoordinating solvent, such as 2-methoxyethyl ether is used, two moles ofH₂ is released per mole of AB complex reacted in a period ofapproximately one day. However, if this same solvent is combined withselected catalysts according to the invention, such as NH₄I, 2.3 molesof H₂ is released per mole of AB complex reacted, in a period of aboutone day. It is desirable to extract as much of the third and last moleof H₂ (per mole of NH₃BH₃ pyrolyzed) as possible. Uncatalyzedthermolytic release of the third mole of hydrogen is known to requiretemperatures in excess of 500° C.

This invention thus overcomes the disadvantages of known techniques fordehydrogenation of ammonia borane (AB) complex by appropriate selectionof dehydrogenation catalysts and solvents, which allow thermolysis orhydrolysis to proceed at comparatively low temperatures, such as 50-80°C. At the lower reaction temperatures provided by the invention,generation of undesirable volatile species that escape the reactor, thatare commonly generated in significant concentrations in disclosedthermolytic systems is minimized. Because such temperatures areavailable by coupling the hydrogen generator to a PEMFC via a suitableheat exchanger, which transfers heat from the PEMFC to the hydrogengenerator, the need for an auxiliary heating device can be eliminated.Alternatively, temperatures in the range of 50-80° C. can be achieved bycombining surrounding and/or coupling (via a suitable heat exchanger)pyrolytic reactors with hydrolytic ones. Applied to NH₃BH₃, theinvention thus lower the dehydrogenation temperature of NH₃BH₃, andallow release of more than two moles of hydrogen per mole of AB complexreacted.

The ammonia borane (AB) complex can be stored dry, mixed with water(slush), or dissolved in water. The catalyst can be added as a solid tothe AB complex mixed (slush) or dissolved in water. The catalyst canalso be dissolved in water and then added to dry AB complex, orslush/dissolved AB in water. Alternatively, the dry AB complex (or anyamine borane complex for that matter) can be added directly to thecocktail that contains the catalyst dissolved in water. Furthermore, thecatalyst can be added at once for faster dehydrogenation of or drop wisefor the controlled hydrogen release from the amine borane complex (e.g.AB complex). Due to the high solubility of NH₃ in water, H₂ gas producedis high purity H₂ gas as defined herein. Boric acid also forms as areaction product, based on XRD and NMR analysis.

There are expected to be many applications for the invention. In oneembodiment, the invention is used to generate H₂₂, which is supplied toa hydrogen fuel cell, such as a PEMFC. FIG. 1 is the schematic of anexemplary electrical power source 100 comprising a hydrolytic hydrogengenerator according to the invention 110 coupled to a PEMFC 160. A heatexchanger 145 receives heat generated by fuel cell 160 in the form ofhumidified oxidant (e.g. air). Heat exchanger 145 transfers heat fromthe PEMFC 160 to the hydrogen generator 110. As a result, system 100does not require an auxiliary heating device to provide heat to drivethe hydrogen generation reaction, such as a reaction temperature of 50to 80° C.

Hydrogen generator 110 shown includes a first compartment 112 holding acatalyst comprising solution and a second compartment holding theammonia borane 113, or other amine borane (AB) complex. Controlelectronics 118 is coupled to catalyst mass flow controller 119 andhydrogen mass flow controller 120. Catalyst mass flow controller 119controls the flow of the catalyst solution, which enters secondcompartment 113 to achieve a desired hydrogen flow generated by hydrogengenerator 110. Coupling connector 132 delivers hydrogen generated byhydrogen generator 110 to the anode of PEMFC 160.

In the embodiment shown in FIG. 1, the AB complex is stored in secondcompartment 113 as aqueous slurry (AB mixed with water). In operation,as soon as the hydrogen generator 110 is turned on, the controlelectronics 118 sends a signal to a mass flow controller (or a flowcontroller) 119 allows a predetermined flow rate of catalyst comprising“cocktail solution” to flow into the second compartment which holds theAB slurry. As a result, hydrogen gas in generated. Both the boric acidand ammonia reaction by-products are captured and remain in the secondcompartment 113. As noted above, although not shown in FIG. 1, inalternate embodiments the ammonia borane complex can be provided in dryform, or be in a dry state or as a slurry mixed with water or diglymeand pumped into a second compartment 113 holding the catalytic cocktail.

Hydrogen generators disclosed herein are capable of delivering PEMFCgrade hydrogen gas by virtue of the low reaction temperature available,safely and reliably in an integrated and self-sustaining device thatoffers high specific energy storage density. Hydrogen PEM fuel cells areoptimal for applications in the power range of 5-500 W where batteriesand internal combustion engines do not deliver cost-effective andconvenient power generation solutions. Unlike secondary batteries, thehydrogen generators disclosed here and formulations therein provide aconstant source of power in a compact size that does not requireelectrical recharging.

System 100 shown in FIG. 1 and related systems can thus provide a viablesolution to many military and civilian applications in need of alightweight, highly dependable power source. Examples include, amongothers, auxiliary power units for small and remote applications, shelterpower, emergency power, external power pack, battery charger, portablepower for soldiers, unmanned aerial vehicles, and robotics.

EXAMPLES

It should be understood that the Examples described below are providedfor illustrative purposes only and do not in any way define the scope ofthe invention. In all cases, AB (90% technical grade) was used asreceived from Aldrich Chemicals, Co. Examples 1-24 describe hydrogengeneration via hydrolytic dehydrogenation of amine borne complexes, ingeneral, and ammonia borane complex, in particular. Examples 25-30descnrbe thermolytic dehydrogenation of the ammonia borane complex. Inmost of the hydrolytic Examples provided, >80% hydrogen yield wasobtained in <10 minutes. In certain Examples, >90% hydrogen yield wasobtained in <5 minutes.

Example 1

To 0.1 g (3.2 mmol) of AB and 0.2 mL of water, 0.5 mL of 15.4 mMsolution of Cl₆K₂Pt was added at once. The flask was insulated withglass wool. Within 3 minutes, 205 mL of H₂ gas was collected.

Example 2

To 0.1 g (3.2 mmol) of AB, 0.5 mL of 15.4 mM solution of Cl₆K₂Pt wasadded at once. The flask was insulated with glass wool. Within 10minutes, 175 mL of H₂ gas was collected.

Example 3

To 0.1 g (3.2 mmol) of AB and 0.1 mL of water, 0.25 mL of 15.4 mMsolution of Cl₆K₂Pt was added at once. The flask was insulated withglass wool. Within 20 minutes, 185 mL of H₂ gas was collected.

Example 4

To 0.2 g (6.4 mmol) of AB and 0.2 mL of water, 0.5 mL of 19.6 mMsolution (saturated) of Cl₆K₂Pt was added at once. The flask wasinsulated with glass wool. Total of 410 mL of H₂ gas was collected. Tothe same flask, 0.1 g (3.2 mmol) of AB and 0.15 mL of saturated Cl₆K₂Ptsolution was added. Total of 185 mL H₂ gas was evolved. To the sameflask, another 0.1 g of AB and 0.25 mL of saturated Cl₆K₂Pt solution wasadded and total of 215 mL of H₂ gas was evolved. Another 0.1 g of AB and0.2 mL of saturated Cl₆K₂Pt solution was added to the same flask and 200mL of H₂ was released.

Example 5

To 0.1 g (3.2 mmol) of AB and 0.1 mL of water, 0.20 ml of 22 mM solutionof Cl₆Na₂Pt was added at once. The flask was insulated with glass wool.Within 5 minutes, 210 mL of H₂ gas was collected.

Example 6

To 0.1 g (3.2 mmol) of AB and 0.1 mL of water, 0.25 mL of 20 mM solutionof H₂Cl₆Pt was added at once. The flask was insulated with glass wool.Within 7 minutes, 185 mL of H₂ gas was collected.

Example 7

To 0.4 g (12.8 mmol) of AB and 0.2 ml of water, 1.0 mL of 20 mM solutionof H₂Cl₆Pt was added with an injection rate of 0.05 mL/min. The flaskwas insulated with glass wool. Total of 710 mL of H₂ gas was collected.FIG. 2 shows the hydrogen generation results from this Example.

Example 8

To 0.3 g (12.8 mmol) of as received AB (90% technical grade) and 0.1 mLof water, 0.75 mL of saturated solution of K₂Cl₆Pt was added with aninjection rate of 0.02 ml/min. The flask was insulated with glass wool.Total of 570 mL of H₂ gas was generated as shown in FIG. 3.

Example 9

To 0.3 g (12.8 mmol) of as received AB (90% technical grade) and 0.1 mLof water, 0.75 mL of saturated solution of K₂Cl₆Pt was added with aninjection rate of 0.02 mL/min. The flask was kept at 30° C. using an oilbath. Total amount of hydrogen gas generated was 580 mL—as shown in FIG.3.

Example 10

To 0.3 g (12.8 mmol) of as received AB (90% technical grade) and 0.1 mLof water, 0.44 mL of saturated solution of K₂Cl₆Pt was added with aninjection rate of 0.02 mL/min. The flask was kept at 35° C. using an oilbath for the first 5 minutes and then was removed and insulated withglass wool. Total amount of hydrogen gas generated was of 575 ml—asshown in FIG. 3.

Example 11

To 0.1 g (3.2 mmol) of AB and 0.1 mL of water inside a Parr reactor,0.25 mL of 19.6 mM solution of Cl₆K₂Pt was added at once. Within 10minutes, the pressure inside the reactor reached 26 psi and 180 mL of H₂gas was collected.

Example 12

To 0.12 g of AB, 0.25 mL, 0.35 mL, and 0.5 mL of 15.0 mM solution ofCl₆K₂Pt was added at once and the rate of H₂ evolution was monitored.FIG. 4 shows the hydrogen generation results obtained. With 0.35 mL and0.5 mL of catalyst solution, most of the hydrogen gas evolved in lessthan 3 minutes. When 0.25 mL of catalyst solution was used, 93% ofhydrogen gas evolved in less than 9 minutes.

Example 13

A general procedure for the hydrolysis of AB complex using PGM catalystssuch as K₂PtCl₆, (NH₄)₂RuCl₆, Na₃RhCl₆, and (NH₄)₂PtCl₆, is nowdescribed. To 0.05 g of AB complex, 0.1 mL of 19 mM solution of PGMcatalyst was added and the amount of hydrogen evolved was recorded. FIG.5 shows the hydrogen generation results obtained. Ruthenium and rhodiumbased catalysts were found to be the most active, followed by Pt.

Example 14

A general procedure for the hydrolysis of borane tertbutylamine complexusing PGM catalysts such as K₂PtCl₆, (NH₄)₂RuCl₆, (NH₄)₂PdCl₆, Na₃RhCl₆,and (NH₄)₂PtCl₆ is now described. To 0.05 g of borane tertbutylaminecomplex, 0.1 mL of 19 mM solution of PGM catalyst was added and theamount of gas generated was monitored as a function of time. FIG. 6shows the hydrogen generation results obtained.

Example 15

A general procedure for the hydrolysis of various amine boranecomplexes, such as, borane dimethylamine, borane morpholine, and boranetertbutylamine is now described. To approximately 0.10 g of the amineborane complex, 0.34 g of a mixture of the composition: 3.3 g H₂O, 2.5 gH₃PO₄ (85%), and 0.025 g Na₃RhCl₆ catalyst was added at room temperatureand the amount of hydrogen produced was recorded as a function of time.FIG. 7 shows the hydrogen generation results obtained. The catalyst wasmost active toward borane dimethylamine and displayed moderate activityto other amine boranes.

Example 16

Hydrolysis kinetics of AB complex was investigated using ¹¹B-NMR byadding 0.01 g of AB to 7 mL of 5 mM, 10 mM, 30 mM, and 45 mM K₂Cl₆Pt inD₂O solution at 25° C., 30° C., and 35° C. FIG. 8 shows the reactionorder with respect to the catalyst data obtained. FIG. 9 shows theinitial rate of AB hydrolysis using 10 mM K₂Cl₆Pt catalyst solution at30° C.: υ_(o)=k[Cat]^(1.4)[AB].

Example 17

FIG. 10 shows the temperature dependence of the AB hydrolysis reactionrates given by the Arrhenius equation. The temperature dependency of theAB hydrolysis reaction rates was determined to be given by the Arrheniusequation: ln k=ln A−E_(a)/RT, where, activation energy (E_(a)) is equalto: 83.9 kJ mol¹ and pre-exponential factor (A) is equal to: 5.3×10¹⁰ Lmol⁻¹ s⁻¹.

Example 18

A special catalytic mixture (cocktail) was prepared in the followingmanner: 1.5 mL of concentrated H₃PO₄ was mixed with 3.3 mL of deionizedwater and 0.0221 g of Na₃Cl₆Rh−1 mL of cocktail weighed about 1.29 g.Several hydrolysis experiments were carried out using the cocktail ofExample 18 as follows:

Example 19

0.5087 g of AB complex (Aldrich 90%) was placed in an insulated vial and1.2890 g of Na₃RhCl₆-H₃PO₄ catalyst mixture (cocktail), prepared inExample 18, was added at room temperature using an injection pump at arate of 0.025 mL/min. The amount of hydrogen evolution was recorded as afunction of time.

Example 20

0.5129 g of AB complex (Aldrich 90%) was placed in a vial and kept at70° C. oil bath. To the AB, 1.3081 g of cocktail (Na₃RhCl₆-H₃PO₄catalyst mixture), prepared in Example 18, was added using an injectionpump at a rate of 0.025 mL/min. The amount of hydrogen produced wasrecorded as a function of time.

Example 21

0.5028 g of AB complex (Aldrich 90%) was placed in an insulated vial and1.2908 g of Na₃RhCl₆-H₃PO₄ catalyst mixture, prepared in Example 18, wasadded at room temperature using an injection pump at a rate of 0.05mL/min. The amount of hydrogen evolved was recorded as a function oftime. FIG. 11 shows data for this Example along with data from Examples19 and Example 20 (for both Examples 19 and 20; injection rate=0.025mL/min).

Example 22

Three tablets of AB complex (Aldrich 90%) weighing 0.24861 g, 0.25651 g,and 0.25485 g respectively were dropped, one at a time, into an ABhydrolysis reactor maintained at constant 80° C. in an oil bath. Thereactor contained 1.5 mL of a catalytic cocktail mixture with followingapproximate composition: water/phosphoric acid/Na₃RhCl₆salt=57.07/42.57/0.36 percent by weight. The amount and rate of gasgenerated was monitored as a function of time. FIG. 12 shows thehydrogen generation results obtained.

Example 23

A catalytic cocktail was prepared by completely dissolving 0.010 g ofNa₃RhCl₆ into 3.8 mL of methanol. Then, 0.38 mL of this cocktail wassampled and added to 0.10 g of AB complex at 20° C. resulting in theevolution of 195 mL hydrogen gas within 4 minutes.

Example 24

The exemplary hydrolytic hydrogen generator 110 shown in FIG. 1 wasconfigured and tested. For the purpose of this Example, design of thehydrogen generator was based on the following metrics: hydrogen flowrate of 340 standard mL/min corresponding to the PEMFC nominal power of30 Watts (net)—this corresponds to about 30.6 mg/min of hydrogen (or61.3 Watts of hydrogen, thermally, based on LHV of H₂; or a PEMFCefficiency of about 48.9%). The hydrogen stoichiometry for thehydrolysis of AB complex is as follows:

NH₃BH₃+3H₂O+⅓H₃PO₄+Na₃RhCl₆ catalyst→⅓(NH₄)₃PO₄+B(OH)₃+3H₂

Ammonia borane (AB) complex reacts with a cocktail formed by-mixingwater, phosphoric acid (an ammonia sequestering agent) and Na₃RhCl₆ salt(a catalyst) with the following composition:H₂O/H₃PO₄/Na₃RhCl₆=62.0%/37.6%/0.4% by weight. Density of the cocktailis about 1.34 g/mL. The stoichiometry above requires that approximately2.1 mL (2.8 g) of cocktail should be added for each gram of AB complexreacted. In order to generate 340 standard mL/min of H₂, 0.469 g/min ofAB should react with 0.985 mL/min (1.313 g/min) of cocktail solution.

As noted above, there are two basic methods for designing the hydrolytichydrogen generator. In one embodiment, a given quantity of dry ammoniaborane complex is placed within a holding tank/reservoir. Another tankholds the cocktail solution. As soon as the unit is turned on, thecontrol electronics send a signal to a mass flow controller (or a flowcontroller) which then allows 1.313 g/min (or 0.985 mL/min) of cocktailsolution to flow into the AB reservoir—resulting in the generation andrelease of 340 standard mL/min of hydrogen gas. Both the boric acid andammonia will be sequestered and remain in the AB reservoir.Alternatively, the AB complex may be stored in a holding tank as anaqueous slurry (AB mixed with a suitable agent that does not promotedehydrogenation of the AB complex such as water (if the mixture alwaysstored at low, near ambient temperatures), or compounds such as higheralcohols (e.g., 1-buthanol) instead of dry state. It is also possiblethat the cocktail solution is placed within a holding tank/reservoir andAB complex, in a dry state or as slurry or paste formed by mixing ABwith water (if the mixture remains at near room temperatures) or with anauxiliary agent such as 1-buthanol, is pumped into the tank holding thecatalytic cocktail. In this case, 0.469 g/min of AB complex needs to beadded or pumped into the catalytic cocktail holding tank in order togenerate 340 standard mL/min of high purity (greater than 99.99% byvolume) hydrogen gas.

It is also possible to design the hydrogen generator in such a fashionso that pellets or tablets or pastes made with the AB complex are addedto the catalytic cocktail mixture in order to generate high purity(greater than 99.99% by volume) hydrogen gas. As such, a device can beconfigured and designed so that it holds the AB complex introducingsmall portions of it (in the order of about 0.261 g) intermittently(i.e., one portion or tablet every minute in order to produce enough H₂to operate a PEMFC and generate 30 W of electrical power) into areservoir or tank holding catalytic cocktail mixture prepared by themethod of Example 18 in a manner similar to that described in theExample 22.

Examples 25-30 below describe hydrogen generation via thermolyticdehydrogenation of ammonia borane complex. Examples 25 (no solvent andno catalyst) and 26 (no catalyst) are controls.

Example 25

0.206 g quantity of NH₃BH₃ (Aldrich 90% Technical grade) was weighedinto a 10 mL glass vial that was attached to polyethylene tubingcontaining a syringe sample port and a water displacement system forquantitatively measuring the amount of H₂ produced. The vial wasimmersed in a 70° C. oil bath. No hydrogen gas was generated even afterfive days.

Example 26

0.210 g of NH₃BH₃ was weighed into a 10 mL glass vial and 0.4-mL ofiso-octane added. The mixture was reacted as in Example 25. After 23hours at 70° C., 92 mL of hydrogen (0.67 moles) was collected.

Example 27

0.205 g of NH₃BH₃ was weighed into a 10 mL glass vial and 0.4 mL of2-methoxyethyl ether added. The mixture was reacted as in Example 25.After 23 hours at 70° C., 265 mL of hydrogen (2.0 moles) was collectedas shown in FIG. 13.

Example 28

0.204 g of NH₃BH₃ was weighed into a 10 mL glass vial and 0.4 mL of2-methoxyethyl ether, and 0.0405 g NH₄I added. The mixture was reactedas in Example 25. After 22 hours at 70° C., 305 mL (2.3 moles) ofhydrogen was collected—as shown in FIG. 14.

Example 29

0.203 g of NH₃BH₃ was weighed into a 10 mL glass vial and 0.4 mL of2-methoxyethyl ether, 0.0065 g K₃Co(CN)₆, and 0.0074 g NH₄I added. Themixture was reacted as in Example 25. After 26 hours at 70° C., 295 ml(2.2 moles) of H₂ gas was collected as shown in FIG. 15.

Example 30

0.204 g of NH₃BH₃ was weighed into a 10 mL glass vial and 0.4 mL of2-methoxyethyl ether, 0.042 g of NH₄I, 0.0029 g ofdi-μ-chlorobis(p-cymene)chlororuthenium (II) added. The mixture wasreacted as in Example 25. After 19 hours of run at 70° C., 295 mL (2.2moles) of hydrogen gas was produced.

It is to be understood that while the invention has been described inconjunction with the preferred specific embodiments thereof, that theforegoing description as well as the examples, which follow, areintended to illustrate and not limit the scope of the invention. Otheraspects, advantages and modifications within the scope of the inventionwill be apparent to those skilled in the art to which the inventionpertains.

1. A thermolytic method of generating hydrogen, comprising the steps of: providing an amine borane (AB) complex, at least one hydrogen generation catalyst, and an at least weakly coordinating solvent, and mixing said AB complex, said catalyst and said solvent, wherein hydrogen gas is generated.
 2. The method of claim 1, wherein said hydrogen gas produced by said mixing step exclusive of any separation processing comprises at least 99% by volume hydrogen.
 3. The method of claim 1, wherein said method is performed in a temperature range between 20° C. and 120° C.
 4. The method of claim 3, wherein said method is performed in a temperature range of between 25° C. and 80° C.
 5. The method of claim 1, wherein said hydrogen generation catalyst comprises a transition metal complex.
 6. The method of claim 1, wherein said hydrogen generation catalyst is at least one selected from the group consisting of cobalt complexes, noble metal complexes and metallocenes.
 7. The method of claim 1, wherein said hydrogen generation catalyst is at least one selected from the group consisting of K₃Co(CN)₆ or di-μ-chlorobis(p-cymene)chlororutheniumn (II).
 8. The method of claim 1, wherein said AB complex comprises NH₃BH₃.
 9. The method of claim 1, wherein said at least weakly coordinating solvent is selected from the group consisting of mono, di, tri, and tetraglyme, tetrahydrofuran, dimethylformamide, 1-Methyl-2-Pyrrolidinone, crown ethers and ethylenediamine.
 10. The method of claim 9, wherein said at least weakly coordinating solvent comprises diglyme (2-methoxyethyl ether).
 11. The method of claim 1, wherein said catalyst comprises a salt.
 12. The method of claim 11, wherein said salt comprises a salt of a weak base and a strong acid.
 13. The method of claim 11, wherein said salt is selected from the group consisting of NH4I, NH4Br and NH4Cl.
 14. The method of claim 1, wherein a yield of said hydrogen gas is at least 2 moles per mole per mole of said AB complex.
 15. The method of claim 1, wherein said method is performed exclusive of a support for said AB complex.
 16. A thermolytic method of generating hydrogen, comprising the steps of: providing an amine borane (AB) complex, at least one hydrogen generation catalyst, and an at least weakly coordinating solvent, and mixing said AB complex, said catalyst and said solvent, wherein said method is performed in a temperature range between 25° C. and 80° C. and at least 2 moles of hydrogen gas is generated per mole of said AB complex.
 17. The method of claim 16, wherein said catalyst comprises a salt.
 18. The method of claim 17, wherein said salt is selected from the group consisting of NH4I, NH4Br and NH4Cl.
 19. The method of claim 16, wherein said amine borane complex comprises NH₃BH₃. 